Binding preference of p62 towards LC3-II during dopaminergic neurotoxin-induced impairment of autophagic flux

Accumulating evidence has revealed that autophagy may be beneficial for treatment of neurodegenerative diseases through removal of abnormal protein aggregates. However, the critical autophagic events during neurodegeneration remain to be elucidated. Here, we investigated whether prototypic autophagic events occur in the MN9D dopaminergic neuronal cell line upon exposure to N-methyl-4-phenylpyridinium (MPP⁺), a well-known dopaminergic neurotoxin. MPP+ treatment induced both morphological and biochemical characteristics of autophagy, such as accumulation of autophagic vacuoles and LC3-II form and decreased p62 levels. Further investigation revealed that these phenomena were largely the consequences of blocked autophagic flux. Following MPP⁺ treatment, levels of LC3-II formed and p62 dramatically increased in the Triton X-100-insoluble fraction. Levels of ubiquitinated proteins also increased in this fraction. Further colocalization analyses revealed that the punctated spots positive for both p62 and LC3 were more intense following MPP⁺ treatment, suggesting drug-induced enrichment of these two proteins in the insoluble fraction. Intriguingly, reciprocal immunoprecipitation analysis revealed that p62 mainly precipitated with LC3-II form following MPP⁺ treatment. Transient transfection of the mutant form of Atg4B, Atg4B^C74A, which inhibits LC3 processing, dramatically decreased binding between p62 and LC3-II form. Taken together, our results indicate that p62 can be efficiently localized to autophagic compartments via preferential binding with LC3-II form. This colocalization may assist in removal of detergent-insoluble forms of damaged cellular proteins during dopaminergic neurotoxin-induced impairment of autophagic flux.